1. Field of the Invention
The present invention relates to a method for producing compounds with high latent isocyanate content, and free, non-blocked isocyanate groups and the partially blocked compounds produced according to this method.
2. Brief Description of the Prior Art
Capped isocyanates, also called isocyanate yielders, are long known compounds (Houben-Weyl, Methods of Organic Chemistry XIV/2 S. 61-70, published in 1963). The following are known blocking agents: .epsilon.-caprolactam, tertiary alcohols, phenols, aceto-acetic esters, malonic acid esters, hydrogen, cyanide.
These capped isocyanates have the characteristic of reacting like isocyanates at high temperatures. The deblocking is easier the more acid the hydrogen atom of the masking group is. In contrast to the free isocyanates, mixtures with hydroxyl group-containing substances (and solvents) can be produced with capped isocyanates without causing a reaction when mixed or dissolved at a relatively low temperature, i.e. below the deblocking temperature. Thus, it is possible to produce storage-stable mixtures with products containing hydroxyl groups such as high molecular polyesters, hydroxy acrylates or polyethers with capped polyisocyanates, which do not enter into the desired isocyanate reaction until higher temperatures, i.e. at or above the deblocking temperature of the adducts. They are of considerable importance in the production of caoutchouc elastic products by way of storable intermediate steps, as well as in the production of paints, especially powder paints for metal painting.
Pulverizeable poly-epoxides were the first binding agents to generate interest as the raw materials for powder paints. Phenol-capped isocyanates were first mentioned as new raw materials for powder paints in German DL-PS No. 55 820 (published May 5, 1967, East Germany) in 1965. Powdered mixtures of high melting polyesters containing hydroxyl groups and polyisocyanates whose isocyanate groups are blocked by phenol can be applied to substrates by electrostatic spraying, and hardened through heat to form coatings. The disadvantages of this method are conditioned on phenol separation: strong odor, operating disturbances, particularly caused by the formation of bubbles.
According to the teachings of German DE-OS No. 1 957 483, Federal Republic of Germany, published Nov. 15, 1969 these disadvantages are avoided by using .epsilon.-caprolactam-capped polyisocyanates instead of the phenol-capped compounds mentioned above. However, for many applications the reactivity of the systems described there, of .epsilon.-caprolactam-blocked aliphatic diisocyanates with hydroxyl group-polyesters is not sufficient, not even in the presence of catalysts.